As illustrated in FIG. 9, in a manual transmission for an automobile (including an automated sequential MT that is based on a manual transmission), a transmission gear 1 is rotatably supported by a radial needle bearing 3 around a power transmission shaft 2 (for example, refer to JP 2009-085401 (A)). The radial needle bearing 3 is rotatably provided between a cylindrical outer raceway 4 that is provided around the inner-circumferential surface of the transmission gear 1 and a cylindrical inner raceway 5 that is provided around the outer-circumferential surface of the power transmission shaft 2 with plural needles 6 held by a cylindrical shaped retainer 7 (7a). Engaging teeth 8 for forming a synchronizing mechanism are provided on the side of the transmission gear 1.
A single-piece retainer 7 as illustrated in FIG. 10, or a split retainer 7a as illustrated in FIG. 11 is assembled in the radial needle bearing 3. In either construction, the retainer 7 (7a) is constructed by plural column sections 10 that are spaced around in the circumferential direction and that span between a pair of concentric rim sections 9 that are separated in the axial direction. The spaces that are surrounded on four sides by column sections 10 that are adjacent in the circumferential direction and the rim sections 9 function as pockets 11 for holding the needles 6 so as to rotate freely. As necessary, construction is used in which notch shaped concave sections 12 (see FIG. 9) are provided on the outer end edge section of the rim sections 9 so that it is possible to feed lubricant that is supplied to the inner-diameter side of the retainer 7 (7a) to the outer-diameter side.
In the example in the figure, the transmission gear 1 is located between a stepped section 13 that is formed around the outer-circumferential surface of the power transmission shaft 2 and a synchronizing hub 14 that is fitted with a spline fit with the outer-circumferential surface of the power transmission shaft 2. The synchronizing hub 14 and the engaging teeth 8 form a synchronizing mechanism. When the synchronizing mechanism is disengaged, the transmission gear 1 and the power transmission shaft 2 freely rotate relative to each other, and the transmission gear 1 is in a state that does not contribute to power transmission. On the other hand, when the synchronizing mechanism is engaged, the transmission gear 1 and the power transmission shaft 2 rotate in synchronization, and the transmission gear 1 is in a state that contributes to power transmission. The construction and function of this kind of synchronizing mechanism is conventionally known, and are not related to the scope of the present invention, so a detailed explanation thereof is omitted.
In the case of the retainer that is assembled in the radial needle bearing 3 for a manual transmission for an automobile, using a retainer 7a having a split structure such as illustrated in FIG. 11 compared with a single piece retainer 7 such as illustrated in FIG. 10 is advantageous from the aspect of preventing fretting. A split retainer 7a is constructed by the edges on both ends in the circumferential direction of plural partial cylindrical shaped retainer elements 15 coming in contact with each other or facing each very close. In the example in the figure, the split retainer 7a has a pair of semi-cylindrical retainer elements 15. The retainer 7a is assembled between the outer raceway 4 and inner raceway 5 together with the needles 6, and in a state in which the radial needle bearing 3 is formed, the retainer elements 15 are combined into a cylindrical shape.
The reason that a split retainer 7a is advantageous from the aspect of preventing fretting is that in the case of a split retainer 7a, the retainer element 15 that is in a no-load area are able to be displaced in the circumferential direction an amount equal to the space in the circumferential direction located between the above retainer element 15 and the retainer element 15 that is in a load area, is displaced a little at a time in the circumferential direction due to inertia or vibration caused by rotation. Due to this displacement, the rolling surfaces of the needles held by the retainer elements 15 that are in a no-load area are displaced relative to the outer raceway 4 and inner raceway 5. The split retainer 7a is such that there is this kind of relative displacement, so from the aspect of preventing fretting, it is more advantageous than a single-piece retainer 7 as illustrated in FIG. 10.
JP 2009-085401 (A) discloses an assembly method for assembling a split retainer 7a in a manual transmission for an automobile. In this assembly method, a preliminary retainer 16 as illustrated in FIG. 12 is used. The preliminary retainer 16 has a pair of retainer elements 15a that are formed by injection molding using synthetic resin. Moreover, as illustrated in FIG. 12 and FIG. 13A, the retainer elements 15a are arranged in a shifted state in the axial direction, and plural locations each on the edges of both ends in the circumferential direction of the retainer elements 15a (two locations each in the example in the figures) are connected by small cross-sectional area sections 17. The retainer elements 15a each have a partial cylindrical shape, and have partial arc-shaped raw rim sections 18 that are provided on both ends in the axial direction, and plural column sections 10 that span between the raw rim sections 18. The portions that are surrounded on four sides by column sections 10 that are adjacent in the circumferential direction, and the raw rim sections 18 function as pockets 11 for holding the needles 6 so as to rotate freely.
Each of the small cross-sectional area sections 17 is constructed so that the cross-sectional area is small, and so that when the retainer elements 15a is displaced relative to each other in the axial direction and a force acts in the shear direction, shearing occurs easily. Shear guide sections 20 where the cross-sectional area of the small cross-sectional area sections 17 becomes a minimum are provided in the center positions in the lengthwise direction of the small cross-sectional area sections 17, or in other words, in the center positions in the circumferential direction of the spaces 19 that exist between the edges on both ends in the circumferential direction of the pair of retainer elements 15a. 
As illustrated in FIG. 13A and FIG. 13B, when using a preliminary retainer 16 to assemble the radial needle bearing 3 around the power transmission shaft 2a of the manual transmission for an automobile, first, the needles 6 (see FIG. 9) are held inside the pockets of the retainer elements 15a of the preliminary retainer 16. With the needles 6 held inside the pockets 11, the needles 6 are prevented from coming out from the pockets 11 by stoppers (not illustrated in the figures). With this kind of construction, the preliminary retainer 16 and needles 6 can be handled as a non-divided member. As illustrated in FIG. 13A, the preliminary retainer 16 with needles 6 held inside the pockets 11 is fitted around the outside of the portion of the power transmission shaft 2a that is adjacent to the stepped section 13a. It is also possible to assemble the needles 6 so as to be held inside the pockets 11 after the preliminary retainer 16 has been fitted around this portion.
Next, the transmission gear 1a is fitted around the outside of the preliminary retainer 16 and needles 6, and the components of a synchronizing mechanism such as a synchronizing hub 14a, synchronizer rings 21a, 21b and the like are fitted around the outside of the preliminary retainer 16 and transmission gear 1a on the portion on the opposite side in the axial direction from the stepped section 13a. Then, as illustrated in FIG. 13B, these components 14a, 21a, 21b and the stepped section 13a are brought close together. As a result, the preliminary retainer 16 is pressed on both sides in the axial direction by the stepped section 13a and synchronizing hub 14a, and the small cross-sectional area sections 17 shear at the shear guide sections 20. The preliminary retainer 16 is divided into the retainer elements 15a that are independent from each other, and the phases in the axial direction (left and right direction in FIG. 13B) of these retainer elements 15a coincide. When the small cross-sectional area sections 17 shear, residual protrusions 22 remain on the end surfaces in the circumferential direction of the retainer elements 15a. 
By using a preliminary retainer 16, it is possible to easily perform assembly of a radial bearing that uses a split retainer 7b. Moreover, after assembly, the preliminary retainer 16 forms the split retainer 7b, so when in use, it is possible for there to be relative displacement between the rolling surfaces of the needles 6 that are held by the retainer element 15a that is located in a no-load area and the outer raceway 4 and inner raceway 5. Therefore, using a preliminary retainer 15 is also advantageous from the aspect of preventing fretting. However, when in use and there is relative displacement in the circumferential direction between retainer elements 15a, the tip-end sections of the residual protrusions 22 repeatedly come in contact with the end surfaces in the circumferential direction of the opposing retainer elements 15a. The tip-end section of the residual protrusions 22 may be sharp, or have burrs due to the shearing of the small cross-sectional area sections 17, and there is a possibility that localized wear will occur at the areas of contact between the tip-end sections of the residual protrusions 22 and the end surfaces in the circumferential direction of the retainer elements 15a. As a result, wear fragments that occur due to this wear may enter into the areas of contact between the rolling surfaces of the needles 6 and the pockets 11 of the retainer 7a, and between the rolling surfaces of the needles 6 and the outer raceway 4 and inner raceway 5, which may become the cause of premature flaking of the rolling surfaces of the needles 6, and the outer raceway 4 and inner raceway 5, or may cause a decrease in strength due to a decrease in the cross-sectional thickness of the column sections 10.